Infrared thermography (IRT) has been used experimentally for concrete delamination detection. Past studies were conducted with limited experimental setups, limited conditions, and a lack of comparable IR camera technologies, which make a difference in delamination detection. Thus, there are inconsistencies in the results reported in the literature. In this study, finite element (FE) models of concrete blocks with artificial delamination were developed and analyzed to explore sensitive parameters for the most effective utilization of IRT. After FE model was validated by comparing IRT test results, critical factors of detectability for IRT regarding the size of delamination (area, thickness and volume) were explored by using the FE model. This study can conclude that the most critical factor regarding delamination detection using IRT is the area of delamination; subsequently, the thickness affects the temperature difference of the surface. The volume of delamination by itself is not a significant factor when using IRT. Furthermore, it was found that the effect of delamination size converges to a certain value when the area is 40 × 40 cm and the thickness is 1 cm. This study shows the potential to bring significant improvement for bridge inspection to conduct efficient and effective using IRT.